As a hierarchy multiplexing method, of digital broadcasting, an SDH (synchronous digital hierarchy)/SONET (synchronous optical network) is standardized and a development of an economical digital network progresses.
In a transmission apparatus that has an interface of the SDH/SONET, in order to confirm normality of transmission signals, parity data for detecting a code error is added and a transmission is performed.
In the SDH interface, for example, a frame having VC (virtual container)-3 signal (band: 51.84 Mbps) is configured. On this occasion, a parity of 8 bit (BIP-8: bit interleaved parity code-8) is calculated in units of frame, and its result is inserted into a particular position (B3 byte) of a next frame.
A reception-side transmission apparatus calculates the BIP-8 in units of frame, compares its result and B3 byte of a next frame, and detects a code error (in addition, in the SONET, the SDH corresponding to the VC-3 is referred to as an STS (synchronous transport signal)-1).
For example, when transmitting a frame configured in the order of frames #1 and #2, a transmission-side transmission apparatus inserts a parity calculation result of the frame #1 into the B3 byte of the frame #2 as the next frame for transmission.
When receiving the frames #1 and #2, the reception-side transmission apparatus performs a parity calculation of the frame #1, and compares the calculation result with a value inserted into the B3 byte of the frame #2, thereby detecting the code error of the frame #1.
On the other hand, a transmission system of SDH/SONET has a redundancy configuration of an active system unit and a standby system unit. Further, the transmission-side transmission apparatus is configured by both of an active system unit and a standby system unit. When the active system unit has a breakdown, or maintenance and inspection operations such as a substrate replacement of the active system unit are performed, the transmission-side transmission apparatus switches over to the standby system unit, thereby continuing a communication service.
As a conventional technique, there is proposed a technique in which when detecting a code error in a frame of a reception signal through an active transmission line, in the case where the code error is absent in a corresponding frame of a standby transmission line, a transmission apparatus switches over to the standby transmission line (see, for example, Japanese Laid-open Patent publications No. 07-177116). There is proposed a technique in which a transmission apparatus determines switching based on an error detection signal according to a delay adjustment time of a transmission line and performs transmission line switching (see, for example, Japanese Laid-open Patent publications No. 11-225095). Further, there is proposed a technique on a parity calculation circuit that designates a normal parity calculation range in a first parity calculation after recovered from out of synchronism in a synchronization transmission system (see, for example, Japanese Laid-open Patent publications No. 11-150528).
Signals flowing through a network of the SDH/SONET are transmitted in the form of a multiframe having multiplexed therein a plurality of frames. A transmission-side transmission apparatus inserts a frame head code indicating a frame head into an overhead of a transmission signal for transmission. Then, when inserting the frame head code into the overhead of the transmission signal, the transmission apparatus recalculates parity and replaces parity data as its calculation result.
However, there is the following possibility. That is, when the insertion start timing of the frame head code is different from each other between the active system unit and the standby system unit, the recalculated parity data is different from each other between the active system unit and the standby system unit. As a result, there occurs a phenomenon that the parity data is subsequently mismatched with each other between both of the units.
There is the possibility that under the above-described state, in the case where the switching operation from the active system unit to the standby system unit is performed, when performing a parity check, the reception-side transmission apparatus detects the code error. Therefore, there arises the problem that although a failure does not occur on a transmission line, the transmission apparatus incorrectly recognizes that a failure occurs and disconnects a communication line.